A newly discovered binding site for GMP on the ATP phosphoribosyltransferase subunit will be characterized by dial-GMP covalent labeling experiments. The ability of the enzyme to bind (3H)histidyl-tRNA will be determined as a function of bacterial growth conditions. Nutrient shift-up and shift-down experiments on bacterial strains carrying or deleting the gene for ATP phosphoribosyltransferase will be completed in order to characterize histidine operon regulation by the enzyme. The effect of histidyl-tRNA on the enzymatic activity will be determined.